1. Field of the Invention
The present invention relates to an improved interlayer used in the manufacture of laminated safety glass. More particularly, the invention relates to an interlayer having an improved sheet surface.
2. Description of the Prior Art
Laminated safety glass is well known in the art for use as glazing units for vehicles, especially automobile windshields, and for architectural uses. Laminated safety glass comprises at least one sheet of glass laminated to at least one sheet of a synthetic resin material which is usually referred to as the interlayer. The laminate may be the sandwich type construction wherein the interlayer is interposed between two pieces of glass. Alternately, the laminate may comprise a single sheet of glass laminated to a synthetic resin sheet (also referred to as an interlayer for the purposes of the present invention). In the latter type of laminated safety glass the surface of the synthetic resin sheet, which is not laminated to the glass, may optionally be coated with or laminated to another synthetic polymeric material.
Interlayers are conventionally prepared with rough surfaces, which may be the same or different for both sides of the sheet. The rough surfaces of the sheet are defined herein in terms of peaks and valleys occurring around a roughness mean centerline (hereinafter defined). The rough surfaces are required in order to prevent blocking, i.e., the tendency of the interlayer to stick to itself. Rough surfaces are also required to prevent premature adhesion of the interlayer to the other component of the laminate, usually glass, and to facilitate de-airing of the laminate assembly during the lamination procedure.
Although the surface roughness helps to prevent blocking and facilitates de-airing, it causes problems in the printing of the surface of the sheet. The rougher the sheet the harder it is to obtain an acceptable printed gradient on the sheet. The deeper valleys in the sheet are especially troublesome in that they give rise to significant printing defects. Consequently, seasoning of the sheet, i.e., heating of the sheet at elevated temperatures for prolonged periods of time is required in order to obtain a uniform gradient.
Thus, a need exists in the art for interlayer materials for laminated safety glass having an improved sheet surface which provide a better printing surface while still maintaining the desired level of roughness to minimize blocking and to facilitate de-airing.